Crystal‐Field‐Driven Modulation of Temperature‐Dependent Photophysics in Ce ³⁺ ‐Activated Transparent Ceramics

Ce 3+ ‐doped garnet transparent ceramics provide an ideal model system for elucidating the coupling between lattice structure, crystal‐field strength, and thermal luminescence dynamics in solid‐state emitters. Here, a comparative study of Y 3 Al 5 O 12 :Ce (YAG:Ce) and Lu 3 Al 5 O 12 :Ce (LuAG:Ce) transparent ceramics reveals how lattice contraction and crystal‐field modulation govern their excitation‐dependent behavior. Rietveld refinement and high‐resolution microscopy confirm that replacing Y 3+ with smaller Lu 3 + induces lattice compression and local structural distortion, strengthening the crystal field around Ce 3 + ions. Both ceramics exhibit high transparency (>80% at 600 nm), near‐unity internal quantum yields (98.35% for YAG:Ce, 99.52% for LuAG:Ce), and nanosecond‐scale lifetimes (100.7 ns vs. 86.9 ns). Temperature‐dependent excitation spectroscopy uncovers contrasting trends: the ultraviolet excitation band of YAG:Ce undergoes a red‐shift and intensity quenching with increasing temperature, whereas that of LuAG:Ce remains nearly invariant and even intensifies. Low‐temperature deconvolution identifies reduced Huang‐Rhys factors and weaker phonon coupling in LuAG:Ce, accounting for its superior thermal stability. When integrated into LED devices, LuAG:Ce delivers a luminous efficacy of 117 lm/W at the driving electrical power of 5.74 W, demonstrating outstanding color and thermal robustness. These findings establish a direct structure‐field‐luminescence relationship, offering fundamental guidance for designing thermally resilient, high‐power optical materials.